This invention relates to a process for the production of polyurethane elastomers which exhibit low compression sets. These polyurethane elastomers are prepared by the one-shot process, by reacting a methylene diphenyl diisocyanate having a 2,4'-isomer content of at least 20% by weight, with an isocyanate-reactive component having an average functionality of at least 3.0, optionally, in the presence of additives, at an isocyanate index of 90 to 130. It is preferred that the resultant polyurethane elastomers have a compression set of 10% or less.
Liquid room temperature stable polyisocyanates based on 2,4'- and 4,4'-diphenylmethane diisocyanates containing varying amounts of the 2,4'-isomer are known and described in, for example, U.S. Pat. Nos. 4,118,411, 5,350,778, 5,563,232 and 5,585,452. These liquid polyisocyanates are capable of reacting with a suitable polyol component to form molded products via the one-shot process.
The process of molded articles having a closed surface layer by the isocyanate polyaddition process is known. Such articles may be made, for example, by introducing a reactive (optionally, foamable) mixture based on compounds containing several reactive hydrogen atoms and polyisocyanates into a mold (see, e.g., German Auslegeshrift No. 1,196,864). The compounds with reactive hydrogen atoms typically used are polyethers containing hydroxyl groups. Examples of suitable polyisocyanates include 2,4- and 2,6-toluene diisocyanate, their isomeric mixtures and polyphenyl polymethylene polyisocyanates obtained by aniline-formaldehyde condensation followed by phosgenation. Water and/or fluorinated hydrocarbons may be used as blowing agents. Catalysts known to those in the art to be useful for the production of polyurethanes are generally also used.
Depending upon the starting components (and chain lengthening agents such as glycols or diamines if used), it is possible to obtain both elastic and rigid products and variations between these extremes by this procedure. For molded articles that must withstand heavy wear, it is customary to use slightly branched raw materials because such materials yield a product having elastomer-like characteristics. Molded articles of this kind have been produced on a technical scale (e.g., as shoe soles in the shoe manufacturing industry, bowling balls, encapsulations for transformers, self-leveling flooring compositions for gyms, resins for the encapsulation of air bag sensors, etc.).
U.S. Pat. No. 4,774,263 describes a one-shot process for the production of elastic molded articles having a closed surface layer by the reaction injection molding technique without using an external mold release agent. The reaction mixture used in the '263 patent comprises a di- and/or polyisocyanate in which all of the isocyanate groups are aromatically bound, a polyether, a diamine, and, optionally, auxiliary agents and additives.
U.S. Pat. No. 5,719,229 describes polyurethane elastomers having reduced surface skinning and improved green strength prepared by reaction of polyester-based isocyanate prepolymers with diol chain extenders in the presence of certain surface skinning retardants. Various approaches to eliminate surface skinning and improve green strength have been reported. E.g., U.S. Pat. Nos. 3,806,486, 3,635,906, 4,401,804, 4,551,498, and 4,618,667.
U.S. Pat. No. 5,059,672 describes elastomeric reaction products of an aromatic isocyanate, an aliphatic isocyanate having a functionality of at least two, and an aromatic diamine. Suitable aromatic isocyanates include aromatic isocyanates, aromatic isocyanate terminated aliphatics, aromatic isocyanate terminated cycloaliphatics, aromatic isocyanate terminated quasi-prepolymers, and aromatic isocyanate terminated prepolymers. Suitable aliphatic isocyanates include isocyanate terminated aliphatics, cycloaliphatic isocyanates containing at least two isocyanate groups, isocyanate terminated quasi-prepolymers and isocyanate terminated prepolymers. Chain extenders, fillers, pigments, etc., may also be included.
Various polyurethanes and elastomers which may be prepared from an isocyanate, a chain extender and optionally a polyol, in the presence of various catalysts are known and described in, for example, U.S. Pat. Nos. 4,447,590, 4,523,005, 4,621,113 and 5,208,315, and Canadian Patent 971,184. The catalysts described in these references are standard polyurethane catalysts such as, for example, dibutyl tin dilaurate or dibutyl tin oxide.
U.S. Pat. No. 4,115,429 describes low temperature, stable liquid diphenylmethane diisocyanates which are useful isocyanate components in the manufacturing of one-shot polyurethane elastomers. Also, see U.S. Pat. No. 3,644,457 in this regard.
Low compression set polyurethane elastomers are specifically useful in gasket applications and other applications where dynamic mechanical properties are required, i.e., shock absorbers. Very low compression set polyurethanes can usually only be obtained by the so called "hot cast elastomer process" where isocyanate prepolymers based on MDI with very low NCO content (usually less than 10%) are reacted at elevated temperatures (i.e., 80.degree. C.) with a chain extender (i.e., 1,4-butanediol). The hot melt cast elastomer process does not only require high temperatures but is also expensive and demands equipment with very high accuracy to mix the components due to mix ratios ranging from 90:10 to even 95:5. The one-shot process on the other hand, can be performed at ambient temperatures and allows more robust mix ratios of about 1:1 to about 1:3. However, polyurethane made by the one-shot process as known by the art have not shown the desirable low compression set.